The invention relates to methods for preparing polymer films. Specifically, the invention relates to methods of biaxially orienting high density polyethylene films and the films prepared according to such methods.
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages, including melt film formation, quenching, and windup. For a general description of these and other processes associated with film-making, see K R Osborn and W A Jenkins, Plastic films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
An optional part of the film-making process is a procedure known as xe2x80x9corientation.xe2x80x9d The xe2x80x9corientationxe2x80x9d of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of xe2x80x9corientationxe2x80x9d is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch, and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength.
When a film has been stretched in a single direction (monoaxial orientation), the resulting film exhibits great strength and stiffness along the direction of stretch, but it is weak in the other direction, i.e., across the stretch, often splitting or tearing into fibers (fibrillating) when flexed or pulled. To overcome this limitation, two-way or biaxial orientation is employed to more evenly distribute the strengthalities of the film in two directions, in which the crystallites are sheetlike rather than fibrillar. These biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces, leading to their greater utility in packaging applications.
From a practical perspective, it is possible, but technically and mechanically quite difficult, to biaxially orient films by simultaneously stretching the film in two directions. Apparatus for this purpose is known, but tends to be expensive to employ. As a result, most biaxial orientation processes use apparatus which stretches the film sequentially, first in one direction and then in the other. Again for practical reasons, typical orienting apparatus stretches the film first in the direction of the film travel, i.e., in the longitudinal or xe2x80x9cmachine directionxe2x80x9d (MD), and then in the direction perpendicular to the machine direction, i.e., the lateral or xe2x80x9ctransverse directionxe2x80x9d (TD).
The degree to which a film can be oriented is also dependent upon the polymer from which it is made. Polypropylene, as well as polyethylene terephthalate (PET), and nylon, are polymers which are highly crystalline and are readily heat stabilized to form dimensionally stable films. These films are well known to be capable of being stretched to many times the dimensions in which they are originally cast (e.g., 5xc3x97 by 8xc3x97 or more for polypropylene).
High density polyethylene (HDPE) exhibits even higher crystallinity (e.g., about 80-95%) relative to polypropylene (e.g., about 70%), and HDPE-containing films are generally more difficult to orient biaxially than polypropylene films. U.S. Pat. Nos. 4,870,122 and 4,916,025 describe imbalanced biaxially oriented HDPE-containing films which are oriented up to about two times in the machine direction, and six times or more in the transverse direction. This method produces a film that tears relatively easily in the transverse direction. Multi-layer films prepared according to this method are also disclosed in U.S. Pat. Nos. 5,302,442, 5,500,283, and 5,527,608, the disclosures of which are incorporated herein by reference in their entireties.
British Patent No. 1,287,527 describes high density polyethylene films which are biaxially oriented in a balanced fashion to a degree of greater than 6.5 times in both the longitudinal dimension (i.e., MD) and the lateral dimension (i.e., TD). This method requires a specific range of orientation temperatures.
U.S. Pat. Nos. 4,891,173 and 5,006,378 each disclose methods for preparing HDPE films which requires cross-linking the film, with optional biaxial orientation of the cross-linked film. It is reported that the cross-linking process, which requires irradiation of the film, improves the film""s physical properties. Other cross-linking processes, such as chemically-induced cross-linking, can have similar effects.
U.S. Pat. No. 4,680,207 relates to imbalanced biaxially oriented films of linear low density polyethylene (LLDPE) oriented by being stretched up to 6-fold in the machine direction, and up to 3-fold in the transverse direction but less than in the machine direction.
U.S. Pat. No, 5,241,030 describes biaxially oriented films of a blend of at least 75% of a linear ethylene/alpha-olefin copolymer, but no more than 25% HDPE. The film can be mono- or multi-layered, and can be biaxially oriented, i.e., stretched up to 8:1 in the machine direction, and up to 9:1 in the transverse direction.
U.S. Pat. No. 5,302,327 describes an anti-fogging, heat-sealable polypropylene film. The film includes a polypropylene core and a heat sealable layer of HDPE or ethylene copolymer. These bilayer films can be machine stretched up to 7xc3x97MD, coated or corona-treated to improve wettability, and then stretched up to 10xc3x97TD.
Blown films of HDPE having a ethylene-vinyl acetate heat seal coating used for food packaging but such films must have a thickness of about two mils to meet the water vapor transmission rate (WVTR) requirements for packaging suitable for dry foods such as cereals. Moreover, blown HDPE films do not exhibit the dead-fold properties desirable in food packages, particularly of the bag-in-box type.
In view of the above considerations, it is clear that existing methods for producing biaxially oriented HDPE films yield products which are deficient in desirable physical characteristics. Existing HDPE film-making methods generally require additional chemical components in the HDPE resin (e.g., cross-linking agents) and/or additional process steps (e.g., irradiation). Such limitations not only complicate production, but generally result in increased costs. Moreover, cross-linking tends to lower polymer crystallinity, resulting in higher WVTR and lower stiffness.
Accordingly, it is one of the purposes of this invention, among others, to overcome the above limitations in the production of biaxially oriented HDPE films, by providing an economical and relatively uncomplicated method of making biaxially oriented films which imparts superior characteristics to the films, without requirement for chemical additives such as cross-linking agents, and without requirement for supplemental processing steps such as irradiation of the film.
The present invention is a biaxially oriented high density polyethylene (HDPE) film and a method for making the film. The film includes HDPE having a density of at least about 0.940 (g/cm3), preferably at least about 0.950, and a melt index of from about 0.5 to about 10, which has been stretched in the solid state to a degree of from about 5:1 to about 8:1 in the machine direction and to a degree of from about 6:1 to about 15:1 in the transverse direction, and which includes an orientation imbalance including a greater degree of orientation in the transverse direction than in the machine direction.
Preferably, the biaxiially oriented film is stretched to a degree of from about 6:1 to about 7:1 in the machine direction, more preferably the film is stretched to a degree of from about 6:1 to less than 6.5:1 in the machine direction. It is also preferred that the biaxially oriented film is stretched to a degree of from about 9:1 to about 13:1 in the transverse direction. A highly preferred biaxially oriented film is stretched to a degree of from about 6:1 to about 7:1 in the machine direction, and to a degree of from about 9:1 to about 13:1 in the transverse direction.
The biaxially oriented film preferably includes a base layer of the HDPE and at least one skin layer coextensively adhered thereto. Among the various types of skin layers known in the art, the film preferably includes a heat seal skin layer or an ink-receptive skin layer.
The biaxially oriented film of the invention can be produced as a laminated HDPE film, including a central HDPE base layer and further including an outer casting promoter layer coextensively adhered to a surface of a base layer. Preferably, the film includes a casting promoter layer adhered to each of the major surfaces of the HDPE base layer, providing a three-layer structure. The casting promoter layer generally includes any material, preferably a polyolefin, which promotes casting of the HDPE film. Preferably, the casting promoter decreases the casting temperature required to obtain a high quality cast sheet for the high biaxial orientation process. It is more preferred that, in addition to decreasing the casting temperature, the casting promoter improves the optical properties of the film, including providing increased gloss and reduced haze.
The casting promoter is preferably a polyolefin or a blend of polyolefins. For example, a medium density polyethylene material can be used. Alternatively, a blend of low density polyethylene and HDPE can be used. The casting promoter is preferably a copolymer or terpolymer of at least about 80% propylene and at least one other alpha olefin. More preferably, the casting promoter includes an ethylene-propylene copolymer or an ethylene-propylene-butylene terpolymer including at least about 80% propylene. One highly preferred casting promoter material is an ethylene-propylene copolymer including about 98% propylene and about 2% ethylene. Another highly preferred casting promoter is an ethylene-prdpylene-butylene terpolymer including about 3% ethylene, about 93% propylene, and about 4% butylene.
The casting promoter can be blended with a material which improves heat sealability. Preferably, the material included in the casting promoter is a low density polyethylene or an ethylene-vinyl acetate.
The biaxially oriented film can also be modified by means known in the art, including, for example, coated (in-line or off-line), flame- or corona-treated, or metallized. Moreover, the film can include an antioxidant, filler, particulate, dye, pigment, light stabilizer, heat stabilizer, anti-static agent, slip agent, antiblocking agent, abrasive, or other additive. In, a preferred case, the film has been cavitated during the film-making process:
The invention is also a method of making a biaxially oriented high density polyethylene (HDPE) film, including:
biaxially orienting in the solid state a HDPE sheet including HDPE having a density of at least about 0.940, preferably at least about 0.950, and a melt index of from about 0.5 to about 10, and wherein the biaxially orienting includes machine direction stretching the HDPE sheet to a degree of from about 5:1 to about 8:1, and transverse direction stretching the HDPE sheet to a degree of from about 6:1 to about 15:1,
thereby providing a biaxially oriented HDPE film having an orientation imbalance including a greater degree of orientation in the transverse direction than in the machine direction.
The biaxially orienting preferably includes sequentially stretching the HDPE sheet, by first machine direction stretching the RDPE sheet and then transverse direction stretching the HDPE sheet. Alternatively, the biaxially orienting process can include simultaneously machine direction stretching and transverse direction stretching the HDPE sheet.
The machine direction stretching preferably includes stretching the HDPE sheet to a degree of from about 6:1 to about 7:1 in the machine direction, more preferably including stretching the HDPE sheet to a degree of from about 6.5:1 in the machine direction. The transverse direction stretching preferably includes stretching the HDPE sheet to a degree of from about 9:1 to about 13:1 in the transverse direction. In a more preferred method, the machine direction stretching includes stretching the HDPE sheet to a degree of from about 6:1 to about 7:1 in the machine direction, and the transverse direction stretching includes stretching the HDPE sheet to a degree of from about 9:1 to about 13:1 in the transverse direction.
The method can further include laminating a skin material to the HDPE sheet, such that the skin material is coextensively adhered as a skin layer to a surface of the HDPE sheet, to provide a laminated HDPE sheet. Thus, the method can include depositing a skin layer of a heat seal material onto a surface of the HDPE sheet to provide a laminated HDPE sheet having heat seal properties. Alternatively, the method can include laminating an, ink-receptive material to the surface of the HDPE sheet, to provide a laminated HDPE sheet having enhanced ink retention properties. Other skin layers can also be employed. If a casting promoter is used in the preparation of the film the skin layer can be deposited on a surface of a casting promoter layer to yield a multilaminated structure. The method can include treating the biaxially oriented film to increase wettability and adhesion of coatings, e.g., inks.
The method can further include:
co-extruding the HDPE together with a casting promoter to provide a laminated HDPE co-extrudate, wherein the casting promoter includes a polyolefin having a crystallinity lower than that of the HDPE, to provide a laminated HDPE co-extrudate including a HDPE layer and at least one casting promoter layer, and
casting the laminated HDPE co-extrudate to provide a HDPE sheet for the biaxially orienting.
In a highly preferred case, the co-extruding involves co-extruding the HDPE with the casting promoter to provide a laminated HDPE sheet, having a central HDPE base layer and two outer casting promoter layers coextensive with and separated by the HDPE layer.
According to the method of the invention, the casting promoter is preferably a propylene copolymer or terpolymer including at least about 80% propylene with at least one other alpha olefin. More preferably, the casting promoter is an ethylene-propylene copolymer including about 98% propylene and about 2% ethylene or an ethylene-propylene-butylene terpolymer including about 3% ethylene, about 93% propylene and about 4% butylene. Alternatively, the casting promoter can be a medium density polyethylene or a blend of a low density polyethylene and a high density polyethylene.
The method can further include coating the HDPE film (either in-line or off-line), flame- or corona-treating the film, metallizing the film, or otherwise treating the film to obtain a particular property as desired. Moreover, the HDPE can farther include an antioxidant, filler, particulate, dye, pigment, light stabilizer, heat stabilizer, anti-static agent, slip agent, anti-blocking agent, abrasive, or other additive.
In another embodiment, the invention is a method for making a biaxially oriented high density polyethylene (HDPE) film, including:
a) casting a laminated HDPE co-extrudate including a HDPE base layer and a casting promoter layer, wherein the HDPE has a density of at least about 0.940 and a melt index of from about 0.5 to about 10, and the casting promoter is any material which permits low temperature casting of the co-extrudate to provide a high gauge HDPE sheet suitable for high biaxial orientation; and
b) biaxially orienting the HDPE sheet by stretching the HDPE sheet to a degree of from about 5:1 to about 8:1 in the machine direction and to a degree of from about 6:1 to, about 15:1 in the transverse direction to provide a biaxially oriented HDPE film having an orientation imbalance including a greater degree of orientation in the transverse direction than in the machine direction.
In still another embodiment, the invention is a method for making a high density polyethylene (HDPE) sheet, including:
co-extruding HDPE together with a casting promoter to provide a HDPE co-extrudate, wherein the HDPE has a density of at least about 0.940 and a melt index of from about 0.5 to about 10, and wherein the casting promoter includes a polyolefin having a rate of crystallization and a crystallinity lower than that of the HDPE,
casting the HDPE co-extrudate to provide a high gauge HDPE sheet suitable for high biaxial orientation to provide a biaxially oriented HDPE film.
In yet another embodiment, the invention is a biaxially oriented laminated film structure, including:
a) a base layer including high density polyethylene (HDPE) having a density of at least about 0.940 and a melt index of from about 0.5 to about 10, and
b) an outer layer coextensively adhered to the base material and including a polyolefin having a rate of crystallization and a crystallinity lower than that of the HDPE,
wherein the laminated film structure is high biaxially oriented, including having been stretched to a degree of from about 5:1 to about 8:1 in the machine direction and to a degree of from about 6:1 to about 15:1 in the transverse direction, such that the film has an orientation imbalance wherein the degree of orientation in the transverse direction is greater than the degree of orientation in the machine direction.
In still another embodiment, the method relates to making a cavitated high density polyethylene (HDPE) film. Here the method includes:
a) extruding HDPE having a density of at least about 0.940 and a melt index of from about 0.5 to about 10 and containing therein a cavitating agent to provide a HDPE extrudate;
b) casting the HDPE extrudate to provide a HDPE sheet;
c) biaxially orienting the HDPE sheet by stretching the HDPE sheet to a degree of from about 5:1 to about 8:1 in the machine direction and to a degree of from about 6:1 to about 15:1 in the transverse direction,
thereby providing a cavitated biaxially oriented HDPE film having an orientation imbalance comprising a greater degree of orientation in the transverse direction than in the machine direction.
It has now been discovered that these and other purposes can be achieved by the present invention, which provides a biaxially oriented high density polyethylene film having low water vapor transmission rate (WVTR), excellent gauge profile, high impact strength (relative to monoaxial oriented films), high tensile properties, high stiffness, and other physical properties which are markedly better than blown HDPE films. The films also have dead-fold characteristics which make them well suited for packaging of foods in bag-in-box operations conducted on vertical, form, fill and seal (VFFS) machinery. In other applications, the films of the invention have properties rendering them useful for manufacture of labels, e.g., pressure-sensitive labels, graphic arts materials, and, in general, paper substitutes. The film can beneficially be provided with one or more skin layers. For example, when provided with a heat-seal layer by co-extrusion or coating, the film of the invention is particularly suited for use in packaging, especially of dry foodstuffs. The method for producing the film employs conventional apparatus in a more efficient manner, and does not require chemical cross-linking agents, irradiation, or other complicating production means.
These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention.